1. Field of the Invention
The present invention relates to a self-refresh oscillator for a semiconductor memory device, and in particular to a self-refresh oscillator which can minimize power consumption in a standby mode by sensing a data loss of a memory cell resulting from leakage current, and by controlling a refresh period.
Thus, the self-refresh oscillator in accordance with the present invention can be applied to all semiconductor memory devices which carry out a self-refresh operation.
2. Description of the Background Art
In general, "self-refresh" implies that a dynamic random access memory DRAM itself has a predetermined period and performs a refresh operation in order to maintain a data stored in a memory cell in a standby mode.
As a chip density is increased due to development of DRAM fabrication techniques, high integration DRAMs such as 256 Mb DRAM and 1 Gb DRAM have been developed. Accordingly, a bit number of the DRAM is increased, and thus power consumption during the refresh operation sharply grows. It is because a capacity of a cell capacitor composing the cell is not varied or is reduced, although the number of the cells is increased. Therefore, in case an integration degree of the DRAM is increased by four times, the power consumption during the refresh operation is also increased approximately by four times.
The increase of the refresh power consumption resulting from the increase of the integration degree of the DRAM is one of the main problems in the semiconductor technological development field. Especially in a system provided with a battery such as a notebook personal computer, the power consumption in the standby mode considerably influences on a battery using time.
In the conventional art, an oscillator having a predetermined period is used in the self-refresh mode, and the refresh operation is performed in accordance with the predetermined period. Here, the predetermined period is determined according to a data maintenance time in the DRAM cell at a temperature higher than a normal temperature by predetermined degrees because the data maintenance time in the DRAM cell is very sensitive to the temperature, lasts merely for approximately 0.1 second at a high temperature, although lasting for a few seconds at the normal or low temperature.
Accordingly, the self-refresh period is determined by the very short data maintenance time at a high temperature, and thus becomes shorter. It implies that the refresh operation is often performed. As a result, the power consumption may be increased.
However, the notebook PC which is considerably influenced by the self-refresh power consumption is generally used at the normal temperature. Thus, the conventional self-refresh oscillator determining the refresh period in accordance with the high temperature data maintenance time has a disadvantage in that the power consumption in the standby mode is unnecessarily increased.